Why Buffer Weedicides and How Do You Do It?

Weeds are considered significant threats to natural ecosystems. To the farmer, weeds are also a major threat to farm economics. Weeds interfere with crop growth, choke pastures and may even harm farm animals. Being plants themselves, they compete with crops for soil nutrients and water, leading to poorly growing crops and reduced harvests.

A quick, cost-effective way to combat weed invasions is to apply weedicide. These are dangerous chemicals, however, and you need to provide for adequate protection to prevent the weedicide from drifting to other places in the farm or, worse, to your neighbours.

To limit the extent of weedicide drift on adjacent areas, farmers usually block off an area between the zone to be sprayed and the zones to be protected from drift. This is called the buffer zone. In many cases, the buffer zone consists of vegetation — a stand of tall grasses, shrubs, or trees; sometimes, a strip of paddock left alone without spraying can serve as buffer. It is usually located downwind relative to the sprayed zone, since chemical droplets are wind-borne.

Vegetative buffer zones help localise the drift of sprayed weedicide by catching on their leaves and other plant surfaces (stems, flowers, etc.) the spray droplets carried in the air. There are limits to their filtering capacity as, for instance, when the wind is blowing too strongly; they also cannot capture fine vapours or odours of the chemical.

When establishing a vegetative buffer zone, it is preferable to select plants with numerous surfaces that present small frontal areas to the droplets. Filters normally work better when there is more surface area available to catch the target particles, and the same is true for vegetative buffers. Thus, plants with needle-like foliage and plenty of small branches are better at capturing droplets. Large leaves may still be suitable if the surfaces are covered with small hairs or other protrusions. To guard against having gaps in the lower portions of the vegetative buffer, mixed plantings may be done to have different heights of vegetation.

The buffer zone design should try to achieve the maximisation of surface area for droplet capture and, simultaneously, the minimisation of interference with the direction of airflow around the barrier. The objective of reducing airflow deviation differs somewhat from that of shelterbelts, which seek to change the wind direction away from the leeward side. Having a slight wind passing through the buffer will increase the chances for droplet capture. This argues for a vegetative buffer with adequate porosity to allow the air stream to pass through the barrier instead of above or around it.

Wind experiments have established that a porous buffer is able to capture greater amounts of droplets than a solid buffer. This finding suggests that the height of the vegetative buffer should be significantly higher than the height at which the weedicide spray is released. The rule of thumb is that a buffer should be at least two times higher than the release height. For instance, if one is hand-spraying weedicide at a release height of 1.5 metres, the vegetative buffer should be a minimum 3 metres high.

It has been calculated that the 2:1 ratio in heights provides spray drift protection equivalent to 3-10 buffer heights downwind. More amounts of spray drift will be intercepted when the vegetative buffer is located closer to the release point.